Rhodium is an active metal component used in threeway catalyst systems for the reduction of oxides of nitrogen to nitrogen. Rhodium is selective in the reduction of oxides of nitrogen by carbon monoxide in the presence of excess oxygen. In a condition where there is not an excess of oxygen, rhodium reduces oxides of nitrogen to nitrogen with little formation of ammonia.
Rhodium is a by-product of the mining of platinum. On a world-wide basis, normally about 17 units of platinum are mined for each unit of rhodium. Therefore, the rhodium content of any catalyst system must be used effectively and its effectiveness must be maintained during the operative life of the catalyst system.
As reported in the Journal of Catalysis, Volume 50, pages 407 to 418 (December, 1977), in an article entitled "Surface Interaction in the System Rh/Al.sub.2 O.sub.3 ", rhodium interacts strongly with gamma alumina, which is normally used as a washcoat material for catalyst systems. Under oxidizing conditions at elevated temperatures, rhodium diffuses into the bulk of the gamma alumina. This diffused and dissolved rhodium is only partly recovered when reducing conditions are once again established over the catalyst system. Thus, exposure of a rhodium containing catalyst system, based upon a gamma alumina washcoat to high temperature conditions, results in the loss of rhodium as an effective catalyst material over the life of the catalyst system.
It is a particular object of this invention to provide a catalyst system in which rhodium may be employed as a catalyst and the effectiveness of that rhodium is maintained over the life of the catalyst system.
In our earlier patent application, Ser. No. 943,591 now U.S. Pat. No. 4,172,047 entitled "Catalyst-Rhodium on Alpha Alumina", we taught a catalyst system based on rhodium in which alpha alumina was used as a washcoat, rather than gamma alumina. We taught in that patent application that use of alpha alumina as the washcoat reduced the reaction of rhodium with the washcoat and thereby its subsequent loss as an effective catalyst metal.